linux-kernel - Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

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Date: Thu, 25 Apr 2024 15:30:02 -0500
From: Adam Ford <aford173@...il.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: dri-devel@...ts.freedesktop.org, marex@...x.de, aford@...conembedded.com, 
	Frieder Schrempf <frieder.schrempf@...tron.de>, Inki Dae <inki.dae@...sung.com>, 
	Jagan Teki <jagan@...rulasolutions.com>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman <jonas@...boo.se>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Marco Felsch <m.felsch@...gutronix.de>, Michael Tretter <m.tretter@...gutronix.de>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

On Thu, Apr 25, 2024 at 4:19 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> On 12.02.2024 00:09, Adam Ford wrote:
> > When using video sync pulses, the HFP, HBP, and HSA are divided between
> > the available lanes if there is more than one lane.  For certain
> > timings and lane configurations, the HFP may not be evenly divisible.
> > If the HFP is rounded down, it ends up being too small which can cause
> > some monitors to not sync properly. In these instances, adjust htotal
> > and hsync to round the HFP up, and recalculate the htotal.
> >
> > Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL i.MX8MM with HDMI monitor
> > Signed-off-by: Adam Ford <aford173@...il.com>
>
> Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>

Thank you very much for testing!

>
> > ---
> > V2:  No changes
> >
> > diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> > index 8476650c477c..52939211fe93 100644
> > --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> > +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> > @@ -1606,6 +1606,27 @@ static int samsung_dsim_atomic_check(struct drm_bridge *bridge,
> >               adjusted_mode->flags |= (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
> >       }
> >
> > +     /*
> > +      * When using video sync pulses, the HFP, HBP, and HSA are divided between
> > +      * the available lanes if there is more than one lane.  For certain
> > +      * timings and lane configurations, the HFP may not be evenly divisible.
> > +      * If the HFP is rounded down, it ends up being too small which can cause
> > +      * some monitors to not sync properly. In these instances, adjust htotal
> > +      * and hsync to round the HFP up, and recalculate the htotal. Through trial
> > +      * and error, it appears that the HBP and HSA do not appearto need the same
> > +      * correction that HFP does.
> > +      */
> > +     if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE && dsi->lanes > 1) {

Frieder  &  Marek S,

Marek V is proposing we eliminate the check against the flags and do
it unconditionally.  If I send you both a different patch, would you
be willing to try them on your platforms?  I don't want to risk
breaking a board.
I used the check above from the NXP downstream kernel, so it felt
safe, but I am not as familiar with the different DSI modes, so I am
not sure what the impact would be if this read:

 if (dsi->lanes > 1) {

Does anyone else have an opinion on this?
> > +             int hfp = adjusted_mode->hsync_start - adjusted_mode->hdisplay;
> > +             int remainder = hfp % dsi->lanes;
> > +
> > +             if (remainder) {
> > +                     adjusted_mode->hsync_start += remainder;
> > +                     adjusted_mode->hsync_end   += remainder;
> > +                     adjusted_mode->htotal      += remainder;
> > +             }
> > +     }
> > +
> >       return 0;
> >   }
> >
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>